Disclosed herein is a macro-photoinitiator comprising a photoinitiator covalently bound to a polymer. Also disclosed herein is a low temperature process for the enzymatic polymerization of monomers.
Polyester-based compositions, including low-melt polyester compositions, frequently include photoinitiators that aid in curing of the composition after printing or application to a substrate. However, photoinitiators can migrate or leach from polyester-based compositions such as coatings and toners, both before and after curing. This causes undesirable effects such as (1) discoloration of the composition, (2) loss of adhesion to the substrate, (3) incomplete cure and the like. Additionally, incorporation by blending of photoinitiators into polyester compositions can affect the rheology and glass transition temperature (Tg) properties of the polyester-based compositions prior to curing.
Low-melt polyester-based toners and inks use a combination of amorphous and crystalline polyesters to achieve low-melt behavior, enabling faster print speeds and lower energy consumption. While the melting behavior of polyester-based toners provides advantages over polystyrene-based chemical toners in print speed, fuser life and energy consumption, the synthesis of the polyester resin is time and energy-consuming. In particular, the preparation of polyesters by polycondensation takes several days and relies on high temperatures (T≧190° C.) and low pressures (p≦10 mmHg) to drive the polymerization to completion. Condensation polymerizations also utilize metal based catalysts that are difficult to completely remove from polyester products. These metals reduce the biodegradability of polyesters and so are not desirable.
An additional difficulty with conventional preparation of polyesters by polycondensation is the high temperatures (T≧190° C.) required for preparing polyesters in a polycondensation processes. These high temperatures degrade photoinitiators, and thereby decrease or entirely destroy the reactivity (and thus the effectiveness) of any photoinitiator added.
For example, IRGACURE 2959 is a conventional small molecule photoinitiator available from CIBA Specialty Chemicals used in curing compositions. FIG. 1 is a graph showing the decrease in functionality of IRGACURE 2529 as a function of temperature. As shown in FIG. 1, the degradation of IRGACURE 2529 begins as low as 125° C. and proceeds rapidly to complete degradation as temperature increases above 200° C. During the polycondensation reaction, which regularly takes up to ½ day to complete, a large fraction of the photoinitiator will be degraded.
What is desired is a photoinitiator that is easy to disperse into polymeric materials, forms a stable dispersion in polymeric materials, does not leach out of compositions to which it is added and does not affect the Tg or rheology of polymeric materials to which it is added. Further desired is a low-cost enzymatic polymerization process that eliminates the need for the use of metal based catalysts to enable production of metal-catalyst free polyesters.